U.S. patent number 6,576,886 [Application Number 09/683,825] was granted by the patent office on 2003-06-10 for dynamic control of polarization of an optical signal.
This patent grant is currently assigned to General Photonics Corporation. Invention is credited to X. Steve Yao.
United States Patent |
6,576,886 |
Yao |
June 10, 2003 |
Dynamic control of polarization of an optical signal
Abstract
Techniques for dynamically controlling polarization of an
optical signal by combining both feed-forward and feedback
controls.
Inventors: |
Yao; X. Steve (Diamond Bar,
CA) |
Assignee: |
General Photonics Corporation
(Chino, CA)
|
Family
ID: |
26954172 |
Appl.
No.: |
09/683,825 |
Filed: |
February 20, 2002 |
Current U.S.
Class: |
250/225;
356/368 |
Current CPC
Class: |
G02F
1/0136 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); H01J 040/14 () |
Field of
Search: |
;250/225,216,205,206,214R ;356/453,491,487,364,367,368
;359/281,282,283 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Le; Que T.
Attorney, Agent or Firm: Fish & Richardson P.C.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application
No. 60/270,253 filed Feb. 20, 2001, the disclosure of which is
incorporated herein by reference.
Claims
What is claimed is:
1. A device, comprising: an optical path for transmitting an
optical signal; a polarization controller in said optical path
operable to adjust a polarization property of the optical signal in
response to a control signal; a feed-forward control module
responsive to an input polarization of the optical signal to
control said polarization controller to change the input
polarization to a desired output polarization; and a feedback
control module controlling said polarization controller to reduce a
deviation of said output polarization from said desired output
polarization.
2. The device as in claim 1, wherein said polarization controller
has a plurality of polarization elements cascaded in said optical
path, each polarization element operable to adjust a polarization
property of the optical signal in response to a control signal,
wherein said feed-forward control module is operable to produce
control signals to at least some of said plurality of polarization
elements according to one of a plurality of pre-determined setting
combinations in response to an input polarization of the optical
signal so that said plurality of polarization elements operate in
combination to change said input polarization to a desired output
polarization, and wherein said feedback control module is operable
to produce feedback control signals to adjust settings of at least
some of said plurality of polarization elements to reduce a
deviation of a measured output polarization from said desired
output polarization.
3. The device as in claim 2, wherein each of said feed-forward
control module and said feedback control module is coupled to
control each of said plurality of polarization elements.
4. The device as in claim 2, wherein a polarization element under
control of said feed-forward control module is not controlled by
said feedback control module.
5. The device as in claim 2, wherein a polarization element under
control of said feedback control module is not controlled by said
feed-forward control module.
6. The device as in claim 1, wherein each polarization element is a
birefringent element.
7. The device as in claim 6, wherein each polarization element has
a principal polarization direction fixed relative to a principal
polarization direction of another polarization element and has an
amount of birefringence that is adjustable in response to a
respective control signal.
8. The device as in claim 7, wherein each polarization element
includes an electro-optic material.
9. The device as in claim 7, further comprising a fiber forming
said optical path and wherein each polarization element includes a
fiber squeezer.
10. The device as in claim 6, wherein each polarization element has
a principal polarization direction that is adjustable relative to a
principal polarization direction of another polarization element in
response to a respective control signal and has a fixed amount of
birefringence.
11. The device as in claim 10, wherein each polarization element
includes a rotatable waveplate whose rotation is controlled by a
respective control signal.
12. The device as in claim 10, wherein each polarization element
includes a fiber coil which is rotatable in response to a
respective control signal.
13. The device as in claim 10, wherein each polarization element
includes an electro-optic material whose principal polarization
direction varies in response to a respective control signal.
14. The device as in claim 1, wherein said polarization controller
includes two birefringent wedges that are movable relative to each
other to change a total retardation on the optical signal and that
are rotatable around said optical path.
15. The device as in claim 1, wherein said polarization controller
includes a fiber, and a fiber squeezer operable to apply a variable
pressure on said fiber and to change a direction of said variable
pressure around said fiber.
16. A device, comprising: a plurality of polarization elements
cascaded to define an optical path for transmitting an optical
signal, each polarization element operable to adjust a polarization
property of the optical signal in response to a control signal; a
feed-forward control module operable to produce control signals to
at least some of said plurality of polarization elements according
to one of a plurality of pre-determined setting combinations in
response to an input polarization of the optical signal so that
said plurality of polarization elements operate in combination to
change said input polarization to a desired output polarization;
and a feedback control module operable to produce feedback control
signals to adjust settings of at least some of said plurality of
polarization elements to reduce a deviation of a measured output
polarization from said desired output polarization.
17. The device as in claim 16, wherein said plurality of
polarization elements include four polarization elements, wherein
said feed-forward control module controls two polarization elements
which have respective slow axes rotated from each other by about 45
degrees and produce variable phase retardation values in response
to respective control signals and said feedback control module
controls the other two polarization elements which have respective
slow axes rotated from each other by about 45 degrees and produce
variable phase retardation values in response to respective control
signals.
18. The device as in claim 17, further comprising a fiber forming
said optical path and each polarization element includes a fiber
squeezer engaged to said fiber at a designated location to cause a
variable local birefringence.
19. A method, comprising: measuring input polarization of light
input to an optical path; causing at least some of a plurality of
polarization elements cascaded in the optical path to be controlled
in response to the measured input polarization to rotate the input
polarization to or near a desired polarization; measuring the
output polarization of light from the optical path; and causing at
least some of the plurality of polarization elements to be
controlled in response to the measured output polarization to
reduce a deviation of the output polarization from the desired
polarization.
20. The method as in claim 19, wherein the polarization elements
that are controlled in response to the measured input polarization
are not controlled by the measured output polarization.
Description
BACKGROUND OF INVENTION
This application relates to optical polarization, and more
specifically, to techniques and systems for controlling a state of
polarization of an optical signal.
Various optical devices and systems can be sensitive to the state
of polarization of an optical signal to be processed or
transmitted. For example, certain coherent optical processing may
require a match between the states of polarization of two separate
optical beams when the two beams are superposed. For another
example, a birefringent optical element may attenuate an optical
signal differently when the polarization of the signal forms
different angles with respect to a given principal axis of
polarization of the element. An optical amplifier with a saturable
gain medium may also produce a polarization-dependent gain when a
polarization component with a high intensity saturates the gain
medium and hence experiences an optical gain less than that of
another, weaker polarization component. Furthermore, certain
optical modulators may produce different modulation depths on
optical signals with different polarizations. Semiconductor
electro-absorption modulators and electro-optical modulators based
on birefringent crystals such as lithium niobate are examples of
such modulators. Hence, it is generally desirable to control the
polarization of an optical signal in those and other
polarization-sensitive devices and systems.
The polarization of an optical signal may not be static but
dynamically vary with time in some optical systems due to various
fluctuations or changes in some parts of the systems such as light
sources, optical components, and optical transmission media. For
example, some optical fibers may be birefringent to exhibit
different refractive indices for different polarizations. Typical
causes for this fiber birefringence include, among others,
imperfect circular cores, and unbalanced stress in a fiber along
different transverse directions. Fluctuations in local temperature
and stress along a fiber line, therefore, may randomly change the
axis of birefringence of the optical fiber at different locations.
The polarization of light transmitting through such a fiber,
therefore, may also fluctuate with time and cause polarization-mode
dispersion (PMD) in optical signals with two orthogonal principal
polarization states.
Accordingly, it may be desirable that a polarization control
mechanism be dynamic so that it may change its control in response
to any variation in the input polarization of light and therefore
maintain or set the output polarization at a desired state. Some
dynamic polarization control devices implement an adjustable
polarization module that manipulates the polarization of light, and
a polarization analyzer that measures any deviation of the actual
output polarization from the polarization module from a desired
output polarization. The adjustable polarization module may include
multiple adjustable polarization elements, e.g., rotatable
waveplates or adjustable fiber squeezers engaged to a fiber, to
control the output polarization based on adjustable optical
birefringence. A feedback control loop may be used to control the
polarization elements in the adjustable polarization module to
correct any variations in the input polarization based on the
measured deviation from the polarization analyzer.
SUMMARY OF INVENTION
The present disclosure includes a control mechanism for dynamically
controlling the multiple polarization elements in the adjustable
polarization module by implementing two control mechanisms: a
feed-forward control and a feedback control. In one embodiment, the
feed-forward control measures the input polarization of the input
signal and adjusts the multiple polarization elements to
pre-determined settings for producing the desired output
polarization. The feedback control adjusts the multiple
polarization elements around the settings initially set by the
feed-forward control to reduce the measured deviation of the output
polarization of the adjustable polarization module. In another
embodiment, the feed-forward control is engaged to control at least
two polarization elements while the feedback control is engaged to
control at least two polarization elements that are not engaged to
be controlled by the feed-forward control.
To certain extent, the feed-forward control essentially provides a
fast, coarse control of some or all of the polarization elements in
response to the input polarization and the feedback control
essentially fine tunes the settings of some or all of the
polarization elements to reduce the deviation of the output
polarization from a desired output polarization.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows one embodiment of a dynamic polarization control of
the present disclosure.
FIG. 2 shows one embodiment of the operational flow of the system
in FIG. 1.
FIGS. 3A, 3B, 3C, 4A, 4B, 5A, and 5B show examples of the
adjustable polarization controller in the system of FIG. 1 that has
two or more adjustable polarization elements.
FIGS. 6, 7A, and 7B show exemplary configurations for engaging the
feedback and feed-forward control loops to polarization
elements.
DETAILED DESCRIPTION
The techniques of the present disclosure are in part based on the
recognition that, a feedback control alone may be inefficient or
slow in controlling multiple polarization elements that operate in
combination to control or maintain the output polarization at a
desired polarization. Such a feedback control in some
implementations relies on the output polarization produced by the
multiple polarization elements to adjust the settings of the
multiple polarization elements. In general, a setting in each
polarization element may be adjustable only within a limited tuning
range. Hence, as the input polarization drifts or fluctuates, the
adjustment of one or more polarization elements may reach their
respective limits in an attempt to maintain the output polarization
at the desired state. When this occurs, an out-of-range
polarization element must be reset to a setting within its tuning
range and the combined configuration of the multiple polarization
elements must be readjusted in order to lock the output
polarization at the desired state.
Hence, the above resetting of one or more polarization elements in
the feedback control can be a slow process and therefore, the
output polarization may not be at the desired state during the
reset period. This may be undesirable in a polarization-sensitive
application where a component, device, or system may perform
unsatisfactorily or even fail during the reset period.
It is further recognized that, the above drawback of the feedback
control is partially due to the reactive nature of the feedback
control. The feedback control does not have or does not respond to
information on the input polarization received by the multiple
polarization elements. Instead, it controls the polarization
elements based on the output polarization to correct the error in
the output polarization.
In recognition of the above, the present disclosure implements a
feed-forward control and combines the feed-forward control and the
feedback control to provide a highly-accurate and high-speed
polarization control mechanism. This system may be configured to
essentially eliminate the resetting of the polarization elements by
using the feed-forward control to maintain the setting of each
polarization element within its tuning range. In addition, the
present technique may also be used to mitigate device inaccuracies
such as hysteresis and device parameter drifts due to aging and
environmental factors.
FIG. 1 shows one embodiment of this polarization control system
100. An adjustable polarization controller 120 is used to receive
an input optical signal 104 and to produce an output optical signal
122 at a desired polarization. The polarization controller 120
includes two or more adjustable polarization elements to modify the
polarization of the input signal 104. Each polarization element may
be optically birefringent and use its birefringence to modify the
polarization of light passing therethrough. The polarization
elements may be optically cascaded in series within the controller
120. A feed-forward control loop 101 and a feedback control loop
102 are used to control the polarization elements in the
polarization controller 150.
The feed-forward control loop 101 includes an input polarization
detection module 110 and a feed-forward control circuit 170. The
polarization detection module 110 measures the state of the input
polarization of the input optical signal 104. This module 110 may
be an in-line polarimeter which transmits a received optical signal
and measures its polarization at the same time without altering the
input polarization. Alternatively, the module 110 may be a
polarimeter that destroys the polarization upon measuring the
polarization. Such a polarimeter may be coupled in the system out
of the main optical path to receive a fraction of the input signal
104 by using an optical coupler or splitter in the path of the
input beam 104 to tap the input signal 104 so that the majority of
the input signal 104 continues to propagate to the polarization
controller 120. The module 110 produces an electrical output signal
112 that indicates the state of input polarization of the input
104.
The feed-forward control circuit 170 is coupled to the module 110
to receive the signal 112. It may have a look-up table with
different setting combinations for the polarization elements in the
controller 120 that convert all possible states of polarization of
the input signal 104 into one or more desired polarization states
of the output 122. In each setting combination, the setting for
each polarization element is within its corresponding tuning range
with a sufficient room for adjustment. Preferably, the setting may
be set at or near the middle point of the tuning range to provide a
maximum tuning range. This look-up table may be stored in the
feed-forward control circuit 170.
When the signal 112 is received from the module 112, the
feed-forward control circuit 170 looks up the corresponding
combination setting from the look-up table for converting the
measured polarization indicated by the signal 112 into the desired
polarization. A control command 172 to the polarization controller
120 is then generated by the circuit 170 to set the polarization
elements in the polarization controller 120 according to the
selected setting combination. The look-up table is predetermined
based on calibration and is stored in the circuit 170. The
feed-forward control circuit 170 does not need to perform complex
computation. Hence, the feed-forward control loop 101 can quickly
respond to the measurement in the signal 112 to properly set the
multiple polarization elements in the controller 120.
The above operation of the feed-forward control loop 101 provides a
coarse tuning mechanism for setting the polarization elements in
the controller 120. The feed-forward control loop 101, however,
does not know whether the polarization of the output signal 122 is
in fact at the desired polarization. The feedback control loop 102
is designed to check the polarization of the output signal 122 and
fine tunes the polarization elements to reduce any inaccuracy in
the coarse setting produced by the feed-forward control loop 101and
any variations caused by fluctuations or drifts in the polarization
of the input signal 104.
The feedback control loop 102 includes a polarization detection
module 150 and a feedback control circuit 160. The polarization
detection module 150 may be placed in or out of the path of the
output signal 122. As illustrated, when the module 150 is placed
out of the path of the signal 122, an optical coupler or splitter
130 may be used to tap a fraction of the output signal 122 to
produce a monitor beam 140 as the input to the polarization
detection module 150. A polarimeter or a polarization analyzer
formed of a linear polarizer and a photodetector may be used as the
module 150. A deviation of the polarization of the output signal
122 from the desired polarization is measured by the module 150 and
is included in an error signal 152 to the feedback control circuit
160. The feedback control circuit 160 then initiates an adjustment
routine to fine tune the settings of the polarization elements in
the controller 120 to reduce that measured deviation.
In operation, the feed-forward control 101 resets the settings of
the polarization elements in controller 120 whenever the input
polarization changes based on the pre-determined look-up table.
Hence, a polarization element under the control of the feed-forward
control 101 can be kept within its tuning range without the need
for resetting operation. The feedback control 102, on the other
hand, may operate independently with respect to the feed-forward
control 101 to fine tune the controller 120 and maintain the output
polarization at the desired state.
FIG. 2 shows one embodiment of the operational flow of the
feed-forward control 101 and feedback control 102 shown in FIG. 1.
At step 210, the input polarization is measured. Step 220 selects
from a look-up table a proper predetermined setting combination of
settings for the polarization elements in the controller 120 for
producing a desired output polarization. In implementation, all
polarization elements in the controller 120 may be adjusted to a
predetermined setting combination based on the input polarization.
Alternatively, only two polarization elements in the controller 120
may be set to a predetermined setting combination to control the
polarization to the desired output and the remaining polarization
elements may be controlled according to the polarization output of
the whole controller 120 as shown in steps 240 and 250. At step
240, the output polarization from the controller 120 is measured
and compared to the desired output polarization to determine the
deviation. Based on this measured deviation of the output, all
polarization elements in the controller 120 may be adjusted to
change from the predetermined setting combination to reduce the
measured deviation (step 250). When only part of the polarization
elements in the controller 120 are designed to be adjusted based on
the measured deviation in the output polarization, then only such
designated polarization elements are adjusted in step 250 while
settings of other polarization elements remain unchanged. The above
feed-forward control based on the input polarization and the
feedback control based on the output polarization operate
collectively to control the output polarization.
The multi-element polarization controller 120 may be implemented in
various configurations. FIGS. 3A through 5B show some examples.
FIGS. 3A through 3B show implementations with three cascaded
rotatable waveplates of fixed phase retardation values of 90
degrees, 180 degrees, and 90 degrees, respectively. In FIG. 3A, a
half waveplate (HWP) 312 is placed between two quarter waveplates
(QWP) 311 and 313 in free space to form the controller 120. Two
lenses may be respectively placed at the input and out sides of the
waveplate combination for collimation and coupling to input and
output fibers. FIG. 3B shows a fiber implementation in a fiber 320
where fiber coils 321, 322, and 323 are birefringent to produce the
fixed retardation values of 90 degrees, 180 degrees, and 90
degrees, respectively. Bending of the fiber in each coil introduces
stress and thus causes birefringence. The number of turns (length
of the light propagation) and the diameter of the each fiber coil
(degree of bending) can be set to produce the corresponding fixed
phase retardation. The fiber coils 321, 322, and 323 may be
rotatable as illustrated to change their relative orientations in
their principal axes to adjust the output polarization. I yet
another alternative, electro-optic materials 331, 332, and 333 may
be used to produce the fixed retardation values of 90 degrees, 180
degrees, and 90 degrees, respectively. Electro-optic crystals such
as LiNbO.sub.3 may be used. A waveguide 330 is formed in the
electro-optic materials to direct the light. Unlike the systems in
FIGS. 3A and 3B where each polarization element is physically
rotated for adjusting the polarization, the system in FIG. 3C may
be designed to eliminate physical motion of the polarization
elements 331, 332, and 333. In the embodiment as illustrated, each
electro-optic polarization element is applied with two control
voltages to control and rotate the orientations of their optic axes
via the electro-optic effect without physical rotations of the
elements. This approach may be used to achieve a high-speed
modulation with response times generally shorter than those of the
systems in FIGS. 3A and 3B.
FIGS. 4A and 4B show two implementations of the controller 120
based on the Babinet-Soleil compensator to produce both adjustable
retardation and adjustable orientation. Two movable birefringent
wedges 410 and 420 are positioned in FIG. 4A so that their
hypotenuse surfaces face each other. The input optical beam is
directed to transmit through the hypotenuse surfaces of two wedges
410 and 420. The total optical path length through the wedges 410
and 420 and thus the total retardation of the system may be varied
by moving two wedges 410 and 420 relative to each other. A linear
positioning mechanism may be used to change the relative position
of the wedges 410 and 420. The two wedges 410 and 420 may also be
rotated together about the direction of the input optical beam to
provide the adjustable orientation by a rotation mechanism. In one
implementation of the feedback and feed-forward controls, the
relative position of one wedge with respect to another and the
orientation of the two wedges may be controlled by the feed-forward
and feedback controls 101 and 102.
FIG. 4B shows a fiber polarization controller based on the basic
mechanism of the Babinet-Soleil compensator. A fiber 430 is held to
a base 431. A rotatable fiber squeezer 432 is rotatably engaged to
the fiber 430 on the base 431 so that the direction at which the
squeezer 432 squeezes the fiber 430 can be adjusted. The squeezing
produces birefringence in the fiber 430 to control the light
polarization. A pressure-applying transducer 433, such as a
piezo-electric transducer, may be engaged to the squeezer 432 to
produce a variable pressure and hence a variable birefringence in
the fiber 430. In some implementations, the squeezer 432 and the
transducer 433 may be integrated as a single element. The pressure
to the fiber 430 and the rotation angle of the squeezer 431 about
the fiber 430 may be controlled by the by the feed-forward and
feedback controls 101 and 102.
FIGS. 5A and 5B show implementations of the controller 120 having
four or more adjustable polarization elements with fixed relative
orientations and variable birefringences. In the illustrated
example, the principal polarizations of two adjacent elements are
at about 45 degrees relative to each other. FIG. 5A shows the
implementation with four waveplates 510, 520, 530, and 540 whose
retardation values change in response to external control signals
such as applied voltages. The directions of the same principal
polarization axis such as the slow axis of the four waveplates 510,
520, 530, and 540 may be at 0 degree, 45 degree, 0 degree, and 45
degree, respectively. Electro-optic materials and liquid crystals
may be used. Two collimation lenses may be used to couple the
system to input and output fibers.
FIG. 5B shows an all-fiber implementation in which a fiber 501 is
engaged to four fiber squeezers 512, 522, 532, and 542 whose
squeezing directions are fixed at angles of 0 degree, 45 degree, 0
degree, and 45 degree, respectively. The pressure on each squeezer
may be adjusted to change the retardation produced thereby. Such an
all-fiber design may be used to reduce the optical insertion loss
as compared to other designs and may be used to operate on light of
different wavelengths.
Referring back to FIG. 1, the feed-forward control 101 and the
feedback control 102 may be used in different configurations. In
one embodiment, for example, each adjustable polarization element
in the controller 120 may be controlled by both the feed-forward
control 101 and the feedback control 102. FIG. 6 illustrates one
implementation of this based on the controller shown in FIG. 5B,
where the feed-forward control circuit 170 produces separate
controls signals 172A, 172B, 172C, and 172D in response to the
input polarization to control the polarization elements 512, 522,
532, and 542, respectively, and the feedback control circuit 160
produces separate controls signals 162A, 162B, 162C, and 162D in
response to the measured output polarization to control the
polarization elements 512, 522, 532, and 542, respectively. Hence,
each element is first set by the feed-forward control 101 whenever
the input polarization changes and then is fine tuned by the
feedback control 102 in response to measured output polarization to
reduce the deviation from the desired output polarization.
It is further contemplated that, not every polarization element in
a multi-element polarization controller 120 is controlled by both
the feed-forward and feedback controls 101 and 102. Instead, only
some of the polarization elements may be engaged under the
feed-forward control 101 while other polarization elements are
engaged under the feedback control 102.
For example, in FIGS. 5A and 5B, two polarization elements
respectively orientated at angles 0 and 45 degrees, such as 510 and
520 in FIG. 5A or 512 and 522 in FIG. 5B, may be under the control
of the feed-forward control 101. The other two elements, 530 and
540 in FIG. 5A or 532 and 542 in FIG. 5B, may be under the control
of the feedback control 102. This is because two waveplates capable
of producing variable retardations can produce all possible
polarizations when the same polarization axes (e.g., the slow or
fast axis) orientated at 45 degrees with respect to each other.
Hence, in FIG. 5A, the elements 510 and 520 can rotate the input
polarization approximately at the desired state by the feed-forward
control 101 and the feedback control 102 can fine tune the elements
530 and 540 to accurately set the output polarization at the
desired state. FIGS. 7A and 7B show two implementations of this
embodiment based on the controller shown in FIG. 5B.
Although the present disclosure only includes a few embodiments, it
is understood that various modifications and enhancements may be
made without departing from the following claims.
* * * * *